1. Field of the Invention
This invention relates to an automatic detection method of digital display for measuring the phase and frequency of a pixel clock and an automatic detection apparatus using the method.
2. Description of Related Art
Display is the most important peripheral of personal computer (PC) and shows image frames through display interface card of PC. Based on the pixel clock of PC's display interface card, the analog red, blue, and green light signal sequence of the image frames pixels are serially sent to an analog display for showing text or graphic information. In general, the displaying clock of analog display must be synchronized with the pixel clock of display interface card by using a phase-lock loop (PLL). The display interface card does not provide the pixel clock to the display, and the synchronization of both clocks is hence not easily achieved. The parameters of PLL for synchronization are usually tuned manually according to experience of user or assembly person, and this object is very laborious and difficult. Recently, an automatic detection apparatus for measuring the phase and frequency of analog display had been provided. The apparatus captures and transfers the image of display with a video camera to electric signals, and then transmits these signals to the adjusting unit thereof through a RS232 bus. The adjusting unit of apparatus tunes the displaying clock's phase and frequency of PC display according to the captured image. The drawback of automatic detection apparatus of prior art is that its tuning accuracy and range are lower.
Today, PC displays are digitized. An image frame of digital display consists of a great number of pixels. Based on a pixel clock, the display interface card of a PC serially sends the pixel signals of image frames to a digital display. Based on a displaying clock consisting of a series of sampling instants, the digital display samples the signal sequence of image frame's pixels from the display interface card at each sampling instant. Hence, the above-mentioned displaying clock is usually also called the sampling clock of digital display. If the frequency or phase of displaying clock and the pixel clock are not matched, a signal at a pixel may be displayed at another pixel of digital display. Hence, the image shown by digital display will be distorted, unstable or ambiguous. As shown in FIG. 1 and FIG. 2, the sampling clock 104 of digital display and the pixel clock 102 of PC's display interface card have the same phase and frequency. The frequency or phase of displaying clock 104 is faster than the pixel clock 101 of PC's display interface card, and the frequency or phase of displaying clock 104 is slower than the pixel clock 103 of PC's display interface card. It is obvious that the frequency or phase of both pixel clocks 101 and 102, and the sampling clock 104 are not exactly matched. Hence, the image shown by digital display will be unstable, distorted or ambiguous.